Line and halftone reproduction



Sept. 3, 1946. J. A. c. YULE LINE AND HALFTONE REPRODUCTION Filed May26, 1944 3 Sheets-Sheet l PRog gssso En RECORD {SK G R A ri l NT 2| 22/zo .A 23/ FIGQZAJ 21 ORIGINAL PROCESSED TO MEDIUM HIGH GRADIENT FIG. 2B

FIG.2C. I 33 PROCESSED TO AVERAGE GRADIENT .4

JOHN A.C.YULE

INVENTOR BY WWW Wwwk Sept. 3, 1946.

J. A. c; YULE LINE AND HALFTONE REPRODUCTION 5 Sheets-Sheet 2 FIG.6.

Filed May 26, l944 FIG. 3.

AVERAGE GRADIENT LOG EXPOSURE FIG.5.

PRIOR ART JOHN A C YULE INVENTOR BY W ATT'Y & AG'T p 1946' Y J. A; c.YULE 2,407,211

v LINE AND HALFTONE REPRODUCTION Filed May 26, 1944 3 Sheets-Sheet 3 7C}62 FIG 7A. I

I III-ION TWO TONE ORIGINAF/ SLOW RED- RED I 7 FILTER I 5 Jo FIG. 7D. 5;CONTINUED -g 66 I U I PROCESSING I 64 i I I I J PROCESSED TO E EXPOSEDWHILE 66 "INFINITE" GRADIENT 76 IN DEVELOPER FIG 8.

JOHN A.C.YULE

INVENTOR ATT'Y & AGT

Patented Sept. 3, 1946 LINE AND HALFTONE REPRODUCTION John A. C. Yule,Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., acorporation of New Jersey Application May 26, 1944, Serial No. 537,443

18 Claims.

This invention relates to the reproduction of a two-tone record such asa line drawing or a halftone.

The object of the invention is to reproduce the record without loss ofeither fine light or fine dark details.

A description of the present invention involves certain possiblyunfamiliar terms which are therefore now defined at least in a generalway. Halftones or line drawings such as maps are described as two-tonerecords since they consist fundamentally of dark and light elementswithout any intermediate tones. Of course a map may be made with severalcolors so that contour lines appear brown, rivers appear blue, roadsappear black etc. However, it is still fundamentally a two-tone recordsince it is treated as such in reproduction. For example, a blue filterover such a a record renders the blue tones the same as the light tonesleaving only the other dark tones; or without a filter, the colors canall be reproduced as the dark tone although perhaps not so conveniently,The reproduction of any two-tone original normally involves an infinitegradient process. Photomechanical processes generally come under thisheading. An infinite gradient process is by definition one which willproduce only two tones, a minimum density or light tone and a maximumdensity or dark tone. Any actual process may have a high but stillfinite gamma and this acts as an infinite gradient process because thdifference in density of the two tones being reproduced is such that oneof them gives the minimum density and the other gives the maximumdensity in the reproduction.

The present invention involves unsharp masking, and for completeness,reference is made to my copending application Serial No. 438,633 filedApril 11, 1942. However the earlier application relates primarily to thereproduction of continuous tone originals and the effect of the unsharpmasking in the present infinite gradient processes is different fromthat in continuous tone work. An unsharp mask is one which is made orused slightly out of focus. If the original from which it is made isperfectly sharp, the resulting circle of confusion in the unsharp maskis termed the contribution to confusion. If the original has a circle ofconfusion, the contribution to confusion is just added thereto. Intwo-tone work however both the original and the final reproductionthereof usually have no apparent confusion or unsharpness. Theunsharpness of the mask may be of the disk or rin type or in the case ofhalftone reproduction may have what is termed a squar symmetrycorresponding to the rectilinear distribution of the halftone elementsin the screen pattern.

Since it is not customary in two-tone reproduction to use any form ofmasking whatever, the

normal masking terminology is perhaps unusual but still ha a precismeaning. The masking factor is most easily defined in terms of thedensity range of the two records. For example if the mask has a densityrange only .4 of that of the record it is masking, the masking factor is.4. Density range itself is the difference in density between the lightareas and dark areas of the two-tone record. Usually the mask is madefrom the record it is to mask and in this case the masking factor isnumerically equal to the average gradient" of the mask. Average gradient(see Journal of Optical Society of America, Oct. 1942, vol. 32, p. 558)refers to the contrast of a record relative to some previous, and unlessotherwise specified, the immediately preceding record or scene fromwhich it is made. Gamma is the particular average gradient which relatesto the straight line portion of a characteristic curve. It can be shownthat the average of the gradients at a11 log. exposure values betweentwo given points on a characteristic curve is equal to the slope of thestraight line joining those points which slope is the average gradienfor the exposure range between those points.

In photomechanical or photographic reproduction of two-tone originals itis often difficult to reproduce both fine light and fine dark detailssimultaneously. Proper exposure for the light details (in dark areas)normally causes the dark details (in light areas) to disappear and viceversa. This is exemplified by the Patent Office requirement thatdrawings be made distinctly since the normal reproduction process willotherwise lose the fine details. The reproduction processes hereconcerned always involve at least one infinite gradient step. Forexample an original line drawing or map may be photographed by aninfinite gradient process to produce a negative from which printingplates are made directly. Alternatively the negative may be made at highbut not infinite gradient and the printing plates are then made from thenegative by an infinite gradient process. It has been found that theinfinite gradient step, wherever it occurs, is the one in which eitherthe fine dark or the fine light details are usually lost, although theymay have been already lost at a previous step.

According to the present invention this loss of detail is prevented byunsharply masking the record being printed either at the infinitegradient step or at some prior step. The mask itself may be unsharp ormay be used unsharply with an unsharpness sufficient to smooth out,degrade, or perhaps even obliterate entirely, the finer details thereof.The mask is negative to the record it is masking and should have anefiective masking factor between .2 and .7 preferably between .3 and .5.The record being masked may be the line or halftone original itself inwhich case the resulting negative or any later record may be processedto an effectively infinite gradient. On the other hand the record beingmasked may be a negative of an original, the negative having a densityrange between 1.2 and 3.0 or even 4.0 and the positive mask for thenegative having a density range between .25 and 2.1. A positive may beprinted from the combination and processed to efiectively infinitegradient'or to a lower gradient to be later reproduced at the highgradient.

In simple terms, the principle of the operation of the invention is asfollows. The two-tone recrd consists primarily of two density valuesonly but for various reasons (light scattering, diifraction etc.) finedark details in a region of low density effectively do not have themaximum density and fine light details in a high density regioneffectively do not have the minimum density. In fact the finest darkdetails often have lower efiective density than the finest lightdetails. Any actual paleness of the dark details increases this effect.The mask according to the present invention reduces the over allcontrast of the record but has little or no effect on the contrast offine details. This raises the density of the light regions including thedensity of the dark details until the dark details have a densitygreater than the light details. Precise masking would raise the darkdetails to the maximum density exactly and also bring the light regionor minimum density up to that of the light details. With such anarrangement it is easily possible to produce all details both light anddark by any infinite gradient process.

For clarity, the following example is included. A two-tone record ismade up primarily of areas of density 0.2 and areas of density 2.0.However the efiective density of fine dark details in light regions isonly 0.8 (not 2.0) and that of fine light details in dark regions is 1.4(not 0.2). No infinite gradient step can simultaneously record both setsof details since the dark Ones have lower density than the light ones.Normal gradient processes can of course record all the details butcannot make the dark detail density greater than the light detaildensity and hence can never be suitable for infinite gradientreproduction. The mask according to the present invention has in thiscase a density of 1.3 in the highlights and 0.1 in the shadows (the maskis negative to the record). Total shadow density is now 2.0+0.l (2.1);the highlight density is 0.2-|1.3 1.5); the fine light details, havingpractically the same amount of masking as the shadows because of theunsharpness, have a density 1.4l+0.1 (1.5) and the fine dark detailsreceive the same masking as the rest of the highlight region and have adensity 0.8+1.3 (2.1). Thus fine details have the same densities asother regions of the same intended tone and can be reproduced easily.

In the reproduction of line drawings I have found that the contributionto confusion should preferably be between .003 and .03 inch so that onthe one hand it is effective and on the other hand the unsharpness isnot apparent as objectionable outlining in the final print. The upperlimit (.03 inch) and up to .05 inch in the fine type of work to whichthe invention is applied gives a detectable outlining of coarse detailssuch as lettering so that line lines appear in the reproductionterminated just before they cut the letters: This enhances thelettering. The lower values of unsharpness give precise reproduction ofthe original. On the other hand when the two-tone record being masked isa halftone the contribution to confusion in the mask or in the way it isused should be of the same order of size as the halftone element.Preferably the unsharpness should have a square symmetry and be equal tothe size of the halftone element so that each light area in the halftoneoriginal contributes to the production of four dark areas in the maskwhich are located to mask the corresponding four dark areas in theoriginal.

Another way of describing the operation of the invention is in terms ofthe difference in exposure which would be required without masking toreproduce fine light details in one case and fine dark details inanother. For example, fine light details in an original might requirefour times as much exposure for proper reproduction as is required forgood reproduction of the fine dark details. That is, the properreproduction would require exposures differing by a factor of four. Ifthe logarithm of this factor is D, the unsharp mask according to thepresent invention should have a density range approximately equal to D.With a factor of 4, D is equal to 0.6 and the mask should have a densityrange roughly between 0.5 and 0.7. The effect of the mask is to reducethe contrast of the record, but this is immaterial since it is beingreproduced by an effectively infinite gradient process anyway; moreoverthe mask, due to its unsharpness, does not reduce the contrast of thefine details but only insures that they shall receive the correctrelative exposure, therefore both the fine light details and the finedark details are reproduceable by the infinite gradient process. It willbe realized that the present invention thus provides an automaticdodging in place of the inefiicient manual dodging often adopted whensuch problems of differing exposures arise. Masking by too high a factortends to give an unnatural lightness to dark areas containing lightdetails.

Preferably the mask is held integrally in register with the record evenwhen being processed, to avoid the need for careful registration later.

' Of course a certain amount of unsharpness renders the registrationsomewhat simpler but integral masking is preferable, especially when theabove-mentioned square type of symmetry is employed in halftoneproduction. With this square symmetry, registration of the mask and therecord manually would be extremely difficult.

In any emodiments of the invention, it is possible to get at least partof the effect desired by making only the major portion of the printingstep in question, through the mask, the remainder of the exposure beinggiven without the mask. For most purposes however I prefer to make allof the exposure unsharply masked.

Fig. 1 represents one form of the fundamental step of the invention.

Figs. 2A, 2B and 2C constitute a flow chart illustrating several stepsin one embodiment of the invention.

Fig. 3 is included to aid in the definition of certain technical termsused.

Fig. 4 is intended to represent an enlarged detail of an original map.It is drawn greatly enlarged since at normal size, the fine detailswould be objectionable to the Patent Oifice whose reproduction processeswould not adequately reproduce them.

Fig. 5 is a similar enlarged detail of a reproduction of the map shownin Fig. 4 by prior processes Fig. 6 is similar to Fig. 4 illustratingreproduction according to the present invention.

Figs. 7A to 7D constitute a flow chart illustrating several steps of anembodiment of the invention involving integral masking.

Fig. 8 shows an enlarged detail of a halftone original to be reproduced.

Fig. 9 illustrates the step of making an unsharp mask with squaresymmetry from a halftone original.

Fig. 10 shows the combination of the original shown in Fig. 8 with anunsharp mask having square symmetry.

In Fig. l a two-tone record II which may be an original or a positive ornegative photograph of an original and which may be a line drawing or ahalftone record is unsharply masked by a mask I2 which is negative tothe record. By means of a lens I3 the masked combination is printed ontoa sensitive layer M which is then processed to an effectively infinitegradient. That is, it is processed so that the light areas in theoriginal record appear at maximum density in the print and the darkareas appear at minimum density with no intermediate densities beingformed in the layer I4. The mask I2 may be either unsharp itself orspaced from the record so as to act unsharply. Theoretically the maskcould be adjacent; to the sensitive layer rather than to the record butregistration difficulties in this case would be greater. The effectiveunsharpness of the mask may be measured either at the record II or atthe sensitive layer I4. The masking factor should be between .2 and .7preferably between .3 and .5 expressed in another way between D0.l andD+0.l where D is the logarithm of the factor by which the exposureswould differ in properly reproducing the fine light details and the finedark details of the record I I without the mask I2. Alternately thelayer I4 may be processed by reversal to be positive relative to therecord II. Also it may be processed to a lower but still high gradientand thus contain the fine light and fine dark details emphasized so asto be easily reproducible by a later infinite gradient step. The lens I3is unnecessary of course when contact printing is employed.

The process may involve relatively low gradient steps prior to theinfinite gradient step in which case the unsharp masking according tothe present invention is not needed until the. infinite gradient step isreached but may be used any time prior thereto. This is illustrated inFigs. 2A and 2C in which an original map or other line drawing 20 isilluminated by lamps 2| and printed through a lens 22 onto a medium highgradient negative record 23. The record 23 is then positioned as shownin Fig. 2B slightly out of printing relation with a sensitive layer 26which is ultimately to become an unsharp mask for the negative 23. Aring source of light provided by a, lamp 2'! and a circular transparentarea 28 in an opaque mask 29 exposes the sensi- 6 tive film 26 throughthe negative 23. The inn sharpness or diffusion in the positive 26 isringshaped which has been found to be preferable for most purposesalthough the difference in the effects of various forms of unsharpnessare relatively minor.

The positive 26 is then processed to an average gradient of .4 whichmeans that it is processed to have a density range of .4 of that of thenegative 23. When printing from the masked combination 23 and 26 asshown in Fig. 26 the slight magnification of the positive 26 relativeto' the negative 23 produced by the step shown in Fig. 2B is taken intoaccount and compensated for by placing the positive behind the negativewhen printing therefrom. That is, the distance from the light source 28to the negative 23 in Fig. 213 should be approximately the same as thedistance from the negative 23 to the lens 3I in Fig. 2C. In the latterfigure a lamp 32 with a light diffusing screen 33 illuminates the maskednegative 23 and through the lens 3| exposes a sensitive layer 35 whichis processed to an infinite gradient and which may be the printing plateitself. That is, the term processing is used to include bothphotographic processing and photomechanical processing.

As a particular example of the Figs. 2A to 20 the negative 23 may be ona high gamma emulsion sold under the trade name Kodalith but since it isto be processed only to medium high gradient, Kodalith developer is notused therewith. Instead it is developed by Kodak Developer D-l 1 forthree minutes. .05 gram of benzotriazole per liter of developer may beadded to reduce fog if desired. Kodalith Halftone film is particularlyuseful and in this case the D-11 developer is perfectly satisfactorywithout the benzotriazole. Alternatively any of the following films orplates, developed in Kodak Developer D-8 for /2 to 2 minutes at 68 maybe used: Kodak Contrast Process Ortho film, Kodagraph Contrast ProcessOrtho thin base film, Kodagraph Contrast Process Ortho plate, KodakContrast Process Panchromatic film, Kodagraph Process Panchromatic thinbase film or Kodagraph C. T. C. Panchromatic plates (these are tradenames). The mask 26 is made on Kodak Commercial film or on Kodak 33plates developed to a gamma of about .7. This gives an average gradientof .4 as will be explained in connection with Fig. 3 due to the long toeon the characteristic curve. Four minutes development in Kodak DeveloperDK-50 at 68 F. is satisfactory. The exposure should be such as to give aclear background but not so that more than the very lightest details ifany are lost. The maximum density should be about .7 to 1.2.

Fig. 3 is included showing two typical H and D or characteristic curvesof photographic emulsions. These are well known, but are included to idin the definition of the terms used in this specification. Fundamentallythere are only two tones, a light tone and a dark tone, in a two-toneoriginal. When printing from this original there is a definite and fixeddifference in exposure from. the two tones. In curve 40, the darkest ofthe two tones of the original or so-called first tone does not givesufficient exposure to affect the film in any way. Therefore the firsttone is reproduced at the minimum density. Similarly the second toneexposure is so great that it reproduces at D max. Obviously the actualslope between D min. and D max. is of no importance and the reproductionis said to have effectively infinite gradient. At this point noconsideration is given to the possibility of fine details appearing asintermediate tones. Of course the total exposure must be controlled sothat the first tone stays at D minimum and the second tone reaches Dmaximum but this leaves considerable latitude in the actual exposure. Alower gradient reproduction process such as is used in processing thenegative 23 between Figs. 2A and 2B, is represented by the curve 4| inFig. 3. If gamma is taken to be the slope of the straight line portionof the curve 4! between points 42 and 43, it will be noted that thegamma is only slightly less than that of the curve 4%. However, due tothe long toe portion :14 the elfective average gradient represented bythe broken line 45 is considerably less than the gamma. In this case thefirst tone reproduces on the toe of the curve somewhat above fog densityand the second tone is reproduced part way up the characteristic curve.In general in two tone work it is satisfactory to discuss gamma,gradient, etc. in terms of density range. The diiierence in densitybetween the first and second tones is said to be the density range andit is obvious that the density range in the reproduction of the twotones on curve 4| is only about half that on curve 40.

The map shown in Fig. i is greatly enlarged (actually 4 times as drawn)compared to the original map since otherwise the fine details would beobjectionable to the Patent Office whose reproduction process beingprior to the present invention necessarily involves the shortcomingswhich can be overcome by the present invention. However by drawing thesection of the mapgreatly (4 times) enlarged, the fine details 56, whichactually are the light areas between contour lines which happen to comeclose together, are still reproducible by normal processes. Similarlfine dark details 5| indicating marsh land and 52 indicating water wouldbe lost if drawn at normal size but can be reproduced in themagnification shown. It will be seen in Fig. 5 that normal re productionof the fine light details involves objectionable merging of the adjacentblack areas.

Similarly the line dark lines are completely or partially lost in thenormal reproduction. In actual maps the relative dimensions of coarseand fine details differ even more than indicated in these drawings. Inthese actual maps, the marsh and water lines were in pale blue and weretherefore even less distinct than they appear in Fig. 4. The lines inquestion are only slightly narrower than (about 60% of) the contourlines, but this narrowness combined with paleness causes them almost todisappear from Fig. 5. The present invention provides reproduction offine dark details whether their normal irreproducibility is due entirelyto fineness or partly to paleness.

Fig. 6 is supposed to be drawn identical to Fig. 4 (except for alettering feature discussed below) to show the reproduction of the mapwhen made according to the present invention. It will be noted thatneither the light nor the dark details are lost. Fig. 6 may beconsidered a black and white reproduction of a colored map representedby Fig. 4.

The lettering 55 in Figs. 4 to 6 is included to illustrate anotherfeature of the invention which is appreciable whe the mask unsharpnesshas a largevalue. It will be noted in Fig. 6 that the coarse detailsrepresented by the bold printing are outlined in white so that finelines terminate before reaching the lettering leaving a space 56.

This enhances the printing rendering it more legible.

In Fig. 7A a two-tone original is shown with six dark elements two ofwhich 6| are close together to enclose a fine light detail. Thisoriginal is illuminated by lamps 62 and is printed through a lens 63onto a blue sensitive emulsion layer 64. This layer is mounted on a filmwith an antihalation inner layer and a slow red sensitive layer 66 onthe back thereof. The antihalation layer may actually be incorporated inthe slow red sensitive layer, or may be separated therefrom, as shownfor clarity.. The antihalation dye and the slowness of the layer 65prevents it being exposed while the blue sensitive layer is exposed. Thefilm is then partially processed to develop the negative in the bluesensitive layer 54 as shown in Fig. 7B. The antihalation dye in thelayer 65 is such that it is removed b this partial processing. Lightfrom a lamp 10 through a red filter H exposes the red sensitive layerthrough the negative 64 without causing any additional exposure of thenegative 64. The processing is then continued increasing the gradient ofthe negative 64 slightly and developing a positive unsharp mask in thelayer 66 with an average gradient such that after thecontinuedprocessing the masking factor is between .2 and .7. It will be notedthat the unsharpness of the elements 12 in the masking layer is suchthat one element 13 thereof covers both images in the negative 64corresponding to the original areas 6!. Thus the mask does not reducethe contrast of the enclosed fine light detail. However there is an overall masking effect according to the factor just mentioned. Light from alamp [5 exposes a photosensitive layer 15 through the maskedcombination. The mask is not only unsharp but it is used unsharply asshown. As before, some compensation may be made for the difference inmagnification between the mask and the negative as in Figs. 2B and 20but for clarity this factor is not considered in connection with Figs.'73 and 7C. Layer I6 is then processed to infinite gradient giving afinal record 11 corresponding to the original 60 and not involving anyloss of fine detail.

The layer 64 may be the emulsion of Kodalith Ortho film and with apartial development between Figs. 7A and 7B of three minutes in KodakDeveloper D-1l (containing .05 gram per liter of antifoggant) reaches anaverage gradient of about 2 or 2.5. The continued processing afterexposure of the red sensitive mask is such as to give an averagegradient of about .4 to the mask. The antihalation dye to preventhalation of blue light and to absorb red light is preferably a graymixture but practically any dye is partially elfective. This dye willprevent exposure of the mask layer 56 when a slow red sensitive emulsionsuch as that on Eastman Spectroscopic Safety Film Type 548 E is used inthis layer 66.

Particularly good dyes for this purpose are mixtures in roughly equalparts of a blue absorber, such asbis(l-p-sulfophenyl-3-methyl-5-pyrazolonel) methine oxonol (U. S.2,274,782 Gaspar) or (B-ethyl-Z-benzoxazole) (1 ethyl 2, 5- dimethyLSpyrrole) -din1ethine-cyanine chloride (U. S. 2,298,731 Brooker andSprague), a green absorber such as bis(l.-p-sulfophenyl-3-methyl-5-pyrazolone-4) trimethine oxonol (Gaspar supra) or acid fuchsin and ared absorber such as his l-p-sulfophenyl-3-methyl-5-pyrazolone-4)pentamethine oxonol (Gaspar supra) or the dye obtained by oxidizing4,4-tetramethyldiamino- 9 4 -methoxytriphenylmethane-3 sulfonic acid (U.S. 2,150,695 Muehler) The integral masking film should preferably bebetween .003 and .03 inch thick. Such spacing combined with the normalscattering of light by the front layer permits an unsharp mask to bemade easily using an extended light source for printing the mask. Inorder to handle two tone work adequately and sharply, the front layershould be one normally developable to an average useful gradient ofabout 2.5 (a range of 2 to 3 being permissible but les than 2 beingtoo'solt especially when 50 per cent masked). For stability andreproducibility, this front layer should reach about 80 per cent or moreof its final gradient. during the first half of said normal development.The rear layer is then exposed through the front and during theremaining half of said. normal development should reach an averagegradient such as to give a masking factor of (a range of .2 to .7 beinguseful). This means that the rear layer exposed to an 80 per cent recordof the final front layer image will make a 50 per cent mask for thatfinal front image if developed to an average gradient of about 0.6. Note.6 .8 2.5 approximately equals .5 2.5. The useful range for the rearlayer gradient is about .2 to .9 which is easily obtainable with commonphotographic emulsions.

In Fig. 8 aflhalftone original (that is, it is considered an original asfar as the present invention is concerned) is made up of shadowwith finewhite details 80, intermediate tones with dark elements 8| andhighlights with fine dark el ments 82. Of course the size of theelements is greatly exaggerated and not intended to be in any exactproportion, but in practice there are some fine light dots and some finedark dots which would be lost in a normal reproduction process. Onemethod according to the present invention for the reproduction of such ahalftone original would involve an unsharp mask in which the unsharpnessis of the same order as the spacing of the elemental halftone dots. Infact such a method is preferable when the mask is separate from theoriginal and the unsharpness may be either of the disk or ring type.However there is one specifically desirable form of unsharpness which isquite practical when an integral masking method is used such as thatillustrated in Figs. 7A to 7D. When the original halftone is on one sideof a film and the mask is made in an emulsion integrally attached to theoriginal, registration is no longer a factor for consideration. As shownin Fig. 9 a halftone negative or positive 85 having an elemental lightarea 86 is on one side of a film 81, the other side of which is providedwith a sensitive layer 88. Lamps 89 are located at the corners of thesquare and are positioned at such a distance from the halftone 05 thatlight from the four lamps through the light area 06 exposes four darkspots on the masking layer 88 which dark spots are located behind thedark areas 90 of the halftone original. Of course each dark area at 90is then masked by a dark area in the mask 88 which is exposed throughfour difierent adjacent light areas of the original. Each light areacontributes partially to the positive masking of the four adjacent darkareas and thus cooperates with the adjacent light areas in theproduction of each of these dark areas in the mask. In fact since darkareas are behind dark areas and light ones behind light ones, there isno negative masking of the elements themselves but there is an over allnegative masking of the rec- 0rd.

This is perhaps best illustrated in Fig. 10 which represents thehalftone negative of Fig. 8 with an integral mask made according to Fig.9 so that dark areas 92 appear in the region of the shadow dots 80, darkareas 93 are superimposed on the dark dots SI of the original and darkareas 94 ar superimposed on the fine dark highlight dots 82 of theoriginal. Since the elements are small, the dark areas of the mask 92are small.

Since the light areas adjacent to the elements 8| are fairly large, thedark areas as superimposed on the elements Bl are fairly large and sincethe dots 82 are small with large light areas surrounding them, themasking elements 94 are quite large. With such an arrangement it hasbeen found that an infinite gradient print may be made from the maskcombination without loss of either the fine light details appearing inthe shadows or the fine dark details appearing in the highlights.

Instead of using four lamps 89 as shown in Fig. 9, four successiveexposures can be given, one corresponding to each of the lamps. Oneconvenient way of doing this is to mount the film 81 on a horizontalturntable rotatable about the center of the film, with a lamp above theturntable and oifset so that each light element of the halftone exposesan area behind an adjacent dark area. Four successive exposures are thengiven, rotating the turntable 90 between each of the exposures.

What I claim as my invention is:

. 1. A photographic process for the reproduction of a minutely detailedtwo-tone original including a series of printing steps, the last one ofthe series being the first infinite gradient step of the process, one ofthe steps of the series consisting of printing a record from a previousrecord which is unsharply masked at least during the major portion ofthe printing'exposure, the mask being negative to the record it ismasking and having an effective masking factor between .2 and .7 and anef fective unsharpness sufiicient to cause the finer details of theoriginal to be substantially lost in the mask but not greater than .06inch.

2. A photographic process according to claim 1 in which said infinitegradient step is the only irefinite gradient one and is the unsharpmasking s p.

3. A photographic process according to claim 1 in which the printing atsaid one of the steps is entirely through the mask.

4. The photographic process for the reproduction of a minutely detailedtwo-tone original comprising printing an effectively infinite gradientrecord from an unsharply masked record, the mask being negative to therecord it is masking and having an effectiv masking factor between .2and .7- and an eifective unsharpness sufficient to cause only the finerdetails of the record to be substantially lost in the mask.

5. The photographic process according to claim 4 in which the recordbeing masked is an original positive and said printing give an infinitegradient negative record.

6. The photographic process according to claim 4 in which the record tobe masked is a negative of an original and has a density range between1.5 and 4, in which said negative is masked by a positiv mask having adensity range between .3 and 1.8 and in which said printing gives aninfinite gradient positive record of the original.

'7. The photographic process according to claim 4 in which the maskedrecord is line work and the mask has a contribution to confusion between.003 and .06 inch.

8. The photographic process according to claim 4 in which the maskedrecord is a halftone record and the mask has a contribution to confusionof the same order of size as the halftone lement.

9. The photographic process according to claim 4 in which the maskedrecord is a dot type halftone and in which the unsharpness of the maskis of the square symmetrical type with an unsharpness equal to thehalftone element to give a masking efiect which is positive for theelemental areas but negative for the over all record.

10. The photographic process according to claim l in which said maskingfactor is between .3 and .5.

11. The photographic process according to claim 4 in which the mask hasa density range between .3 and 1.8.

12. The photographic process according to claim 4 in which the mask hasa density range between .6 and 1.2.

13. The photographic process accordin to claim 4 in which the maskedrecord contains fine dark and fine light details whose respectivereproduction unmasked by infinite gradient printing would requireexposures differing by a factor whose logarithm is D and in which saidmask has a density range approximately equal to D.

14. The photographic process according to claim 4 in which the mask ismade from the record it is to mask and is held integrally in registertherewith while being processed.

15. The protographic process for the reproduction of a fine lineoriginal comprising printing a negative of the original at an averagegradient between 2 and 3 containing both the fine light and the finedark details of the original, printing from the negative a positive masktherefor having an eflective masking factor between .2 and .7,

unsharply masking the negative by said positive and printing an infinitegradient positive from the record so masked, the unsharpness of themasking being suflicient to cause only the finer details of the negativeto be substantially lost in the mask.

16. The process according to claim 15 in which said negative is made ona film containing in addition to the negative emulsion, a sensitiveemulsion spaced from the negative one and onto which the .negative isprinted and which is processed while integrally part of the film.

17. The photographic process for the reproduction of a dot type halftonerecord comprising placing a photosensitive layer slightly out of contactwith the record, exposing the layer by four successive or simultaneousexposures laterally ofiset so as to record the image of each light areaof the record, under each of the four adjacent dark areas of the recordto give a square symmetrical form of unsharpness to the image in thesensitive layer, processing the sensitive layer to an unsharp mask forthe record and printing an infinite gradient record from the masked rec-0rd.

18. The photographic process for the reproduction of a two-tone recordcontaining fine light and fine dark details which if printed by aninfinite gradient process would require for their respectivereproduction, exposures differing by a factor whose logarithm is D,which process comprises making a mask from and for the record, negativeto the record and having a density range approximately equal to D,unsharply masking the record by said mask and printing an efiectivelyinfinite gradient record from the masked record.

JOHN A. C. YULE.

